"So the big question is what kind of benefit is there to tapering for the first set of races?"

None. Well, you'll probably run the half a little faster, maybe even the marathon the next day, too. It will, however, screw up your training schedule, so I can almost guarantee a slower "race" marathon several weeks later.

"Would it be better to just run them as training runs without a taper?"

Yes. If you can handle 39.3 miles over two consecutive days without getting injured or completely killing your legs, it's actually probably a good LSD-style run.

"Is there any reduction in the chance of injury?"

For what? The target "race" marathon? In any event, running 39.3 probably increases your chances of getting injured, both immediately after the back-to-back runs, and possibly also the target marathon--you might cause a nagging injury that is exacerbated a few weeks later.

"Is it good for increasing stored glycogen?"

Yes. This is the big benefit. You should definitely see some increase in glycogen storage capacity.

"I'm not really running any set schedule for training, and will probably just run in the neighborhood of 55 mpw with 20 mile long runs and race off that mileage."

If you think you can do it without getting injured, run the back-to-back runs. Don't race them. Treat them like your big long run before the marathon. Take an easy weekend the next one. Then do a long taper down to the marathon--if you have six weeks between the two, I'd probably do 12, 20, 18, 15, 12, 9, race on the weekends. If it's five weeks, then do 12, 20, 17, 14, 10, race on the weekends.

It's hard to tell you exactly what you should do, since so much depends on your fitness.

If I were in your position, I would start now getting used to doing back-to-back long runs on the weekends, gradually building up. You might start out with 10/10, 10/15, ... until you've worked up to something like 10/20. Those are just examples. But you need to get your body to adapt to that type of pattern of back-to-back long runs. That is also where the improved glycogen storage capacity comes in.

If you check the 'long runs' topic, you'll see the Daniels 'bunching' method on page 2. You might want to give that a try too, doing some double workouts. The post explains how to do it and the benefits of it.

The recovery is key. I'm sure you already know about the 4:1 ratio and all of that. So be sure to get a lot of carbs after each of those training runs.

I would definitely taper, but not a full taper. Perhaps cutback in the week before. And of course, carbo load. There are some excellent tips in the 'fuel' topic. Something that I do is 'top off' my tank the night before a marathon, usually around 11-12 -- even though it might mean having to wake up to do it. It works. I use a high carb drink for that, usually 'Ultra Fuel'. You might also want to experiment with that for your training runs to speed up your recovery. Endurox is also a good supplement to use to help you recover.

As far as pacing treat each run, the half and the full marathon, as a progression run. Start out very slowly and gradually pick up the pace, but not much faster than your long run pace - especially for the half M. That way you're better prepared for the marathon the next day. Same thing. Progression run - no faster than long run pace.

Since your goal is not to race, those runs would serve as training runs, which you've already had plenty of practice with in terms of pacing, fueling, recovering, ... etc. And of course, do a reverse taper after you have completed them so that you will be ready for the marathon 5-6 weeks later. You can also check Hal Higdon's site for tips on doing multiple marathons.

Good luck.